1. Field of the Invention
The present invention relates in general to apparatus for determining the level of material in a container and in particular to a digital point level switch for use with a gamma ray level detection device.
2. Description of the Prior Art
During many manufacturing processes, it is important to know the level of the material in a process vessel. In some cases, a nuclear device is utilized to detect these levels, the device including a gamma ray source and a geiger tube positioned with respect to one another and the vessel such that less gamma rays are received by the geiger tube when the material in the container rises above the level of the source detection line. The geiger tube responds to the gamma rays by generating output pulses.
The conventional prior art point level switch is an analog device which includes an amplifier for amplifying the geiger tube output pulses, a rate circuit for converting the pulses per unit of time into a voltage having a magnitude proportional to the pulse count rate, a trigger circuit for comparing the rate circuit voltage with a reference voltage level to detect the level at which material is to be added to the container and a relay driver and relay for activating an alarm or an apparatus for adding and/or removing the material. In general, these devices must be simple, rugged and inexpensive. In order to operate at minimum possible radiation levels, it is necessary to employ circuitry which is accurate and reliable in an industrial environment. In an analog device, such circuitry tends to be expensive. For example, cost considerations limit the use of precision dial potentiometers for setting the trigger level or levels for the circuit when it is manufactured. Therefore, the device often must be adjusted in the field with the power on under operating conditions. However, under hazardous operating conditions, it may not be possible to make the necessary adjustments. In these cases, the circuitry can be positioned in a location remote from the container, but the cost of the device is increased.
Some attempts have been made to set the trigger levels at the factory, but the settings can be changed accidently during shipment or installation. Furthermore, since point level switches are often required to operate in high vibration applications, the potentiometers for setting trigger levels are vunerable to changed settings and damage.
Another contributor to the vibration problem is the power transformer for the geiger tube high voltage and signal circuit power supply. The weight of this transformer makes effective shockproof mounting of the electronics a problem.